1. Field of the Invention
The invention pertains to a rack and pinion steering system, especially for motor vehicles, with a toothed rack that can be axially displaced in a steering gear housing, with a pinion driven by the steering wheel and actuating the toothed rack and with a spring-loaded pressure part supported in the steering gear housing which serves to generate a certain pressure between the toothed rack and the pinion.
2. Description of the Prior Art
Known rack and pinion steering systems of the type described above have the feature that although when new, the steering torque is constant in all steering positions, with increasing operating time there is a decrease in steering torque in the central steering region (straight-ahead driving), so that the steering becomes much looser in the center region. The reason for this is a reduction in the friction between natural wear. This is because extreme steering inputs are relatively rare during day-to-day operation of the vehicle, so that in practice, the toothed rack becomes more worn in the center region (straightaways and slight curves) than in the positions at each end. As a result of the looseness caused by wear in the center region, the driver feels that the steering is tighter when turning out of this region (in curves).
The above-mentioned problem is a well-known one with rack and pinion steering systems, and there also already exist a variety of suggested solutions aimed at ensuring that the steering force needing to be applied by the driver or at least the subjectively perceived feel during steering actuation, is an uniform as possible in all steering situations. Known measures are essentially based on the idea of raising the pressure between the toothed rack and the pinion in the center region. This purpose is achieved, for example, in a solution disclosed in German Patent DE-OS 24 58 320, by giving the toothed rack a crowned shape at its center.
German Patent DE-OS 33 32 483 discloses the process of adjusting the play between the toothed rack guide and the rear support of the toothed rack so that it is less in the center position of the toothed rack and larger towards the two ends of the toothed rack. In practice, this is achieved--as with the aforesaid DE-OS 24 58 320--by making the toothed rack thicker in its center region.
Lastly, according to German Patent DE-OS 25 26 487, increased pressure between the pinion and the toothed rack in its center region is achieved by a flexible crowning of the toothed rack, associated with direct attachment thereof to the tie rod. Alternatively, it is said that the same can be achieved with an uncrowned toothed rack, although a specifically placed shaft-like springing means would be necessary for the purpose.